Ink jet printers operate by ejecting ink onto a print substrate, such as paper, in controlled patterns of closely spaced dots. By selectively regulating the pattern of ink droplets, such ink jet printers can be used to produce a wide variety of printed materials, including text, graphics, images, and the like. Moreover, ink jet printers are capable of recording permanent images on a wide variety of substrates, including both light reflective and light transmissive substrates.
Ink jet printers typically utilize a variety of inks, including phase change inks, which are often referred to as hot melt inks. Phase change inks are solid at ambient temperatures and liquid at the elevated operating temperatures of an ink jet printing device. Liquid phase ink jet droplets are ejected from the printing device at an elevated operating temperature and, when the ink droplets contact the surface of a substrate, they rapidly solidify to form the predetermined pattern.
Phase change ink is advantageous for printing purposes for a variety of reasons. Problems associated with nozzle clogging due to ink evaporation are largely eliminated, thereby improving the reliability of ink jet printing. Because the ink droplets solidify rapidly upon contact with the substrate, migration of ink along the printing medium is greatly reduced and image quality is improved. The nature and rapid solidification of phase change inks moreover permits high quality images to be printed on a wide variety of printing substrates.
Early references to phase change inks for ink jet printing involved monochrome inks jetted by electrostatic printing devices. Thus, for example, U.S. Pat. No. 3,653,932 discloses a low melting point (30.degree. C. to 50.degree. C.) ink having a base comprising di-esters of sebacic acid. In a similar process, U.S. Pat. No. 3,715,219 describes low melting point (30.degree. C. to 60.degree. C.) inks including a paraffin alcohol-based ink. One disadvantage of printing with low melting point phase change inks is that they frequently exhibit offset problems. Specifically, when substrates printed with these inks are stacked and stored for subsequent use, the ink adheres to adjacent surfaces, particularly if the printed substrates are exposed to high ambient temperatures.
U.S. Pat. Nos. 4,390,369 and 4,484,948 describe methods for producing monochrome phase change inks that employ a natural wax ink base, such as Japan wax, candelilla wax, and carnauba wax, which are subsequently printed from a drop-on-demand ink jet device at a temperature ranging
between 65.degree. C. and 75.degree. C. U.S. Pat. No. 4,659,383 discloses a monochrome ink composition having an ink base including a C20-24 acid or alcohol, a ketone, and an acrylic resin plasticizer. These monochrome ink compositions are not durable and, when printed, may become smudged upon routine handling and folding.
Japanese Patent Application No. 128,053/78 discloses the use of aliphatic and aromatic amides that are solid at room temperature, such as acetamide, as printing inks. U.S. Pat. No. 4,684,956 is directed to monochrome phase change inks utilizing synthetic microcrystalline wax (hydrocarbon wax) and microcrystalline polyethylene wax. This molten composition can be applied to a variety of porous and non-porous substrates using drop-on-demand ink jet application techniques.
Many ink jet printers are capable of discharging multiple ink colors and providing high quality color images. Color ink jet printers typically utilize three base color inks, in addition to black, that are blended together to print a large spectrum of intermediate colors. European Patent Application Nos. 0187352 and 0206286 disclose phase change ink jet printing in color. The ink bases for these systems include fatty acids, a thermoplastic polyethylene and a phase change material in the first application; and the alcohol portion of a thermosetting resin pair, a mixture of organic solvents (o- and p-toluene sulfonamide) and a dye in the second application. Moreover, the development of phase change inks that are substantially transparent provides improved capability to print images on many types of substrates, including light transmissive substrates. Phase change ink compositions disclosed in U.S. Pat. No. 4,899,761 are exemplary.
Phase change ink is conveniently stored, transported, and introduced into an ink jet printer assembly in a solid form. Prior to printing, the ink is heated to a suitable liquid phase temperature. During printer operation, liquid phase ink is supplied to the print head at the proper temperature for ejection. Color ink jet printers use at least one reservoir corresponding to each such color and separate ink jets are in communication with each reservoir for printing the various ink colors. An important consideration in the design of phase change ink jet printers is providing a substantially continuous supply of liquid ink at the ink jet print head from solid ink supply means.
Controlling the supply of phase change ink to the print head is difficult, in part because it requires moderately frequent operator assistance. It would be desirable to provide a phase change ink delivery system requiring minimal operator intervention. Moreover, it is imperative that the correct ink color is provided to each such reservoir and the associated jets. This requires alert operator handling as well as reliable mechanical registration systems to assure that appropriate ink colors remain segregated.
The chemistry of phase change inks also poses challenges to providing a continual supply of phase change ink in the liquid state. It is generally undesirable to heat a large supply of phase change ink or to maintain phase change ink in a liquid state for extended periods of time because extended "cooking" of such inks frequently results in degradation of the ink. Heating of phase change inks is therefore carefully regulated and ink is typically permitted to cool and solidify when the printer is shut off or has been inactive for a predetermined period of time.
Many different arrangements have been devised for supplying phase change ink in a solid form and melting it to supply print head ink reservoirs. For example, U.S. Pat. No. 4,682,185 teaches a spooled, flexible web of hot melt ink that is incrementally unwound and advanced to a heater location. U.S. Pat. No. 4,682,187 teaches delivery of particulate hot melt ink to a melt chamber. Vibration aids gravity feeding of the particulate ink, and the melted ink level, as measured by a float valve, governs introduction of additional ink particulates to the melt chamber.
Several arrangements have been developed in an effort to provide phase change ink delivery systems that reduce operator handling of ink and yet provide a continual supply of ink, in solid form, to the ink reservoir. U.S. Pat. No. 4,609,924 teaches a solid state ink storage means and buffer reservoir that are fixed relative to the ink reservoir located in the scanning print head. The buffer reservoir provides melted ink to the scanning print head on a standby basis.
U.S. Pat. No. 4,870,430 teaches a solid ink delivery system that selectively supplies individual sticks of solid ink to a hot melt ink jet printer head for melting and subsequent printing. Separate ink delivery systems, including separately triggerable ink stick feed assemblies, are provided for each color ink stick delivered to the hot melt ink jet printer. A positioning assembly is provided for aligning the reservoir openings in the print head with the corresponding ink stick feed assemblies. Each ink delivery system furthermore has a registration assembly that prevents triggering of the feed assembly unless the appropriate reservoir opening is properly aligned.
PCT International Publication No. WO 88/08514 teaches a hot melt ink supply system for an ink jet apparatus adapted for use with multiple pigmented inks. Ink is maintained in a liquid condition in two reservoirs: one reservoir is in communication with the ink jet head; and another is a remote supply reservoir. The two liquid ink reservoirs communicate through a flexible supply conduit in which ink is normally maintained in a solid condition. When low ink levels at the print head require transfer of ink, the supply conduit is heated to melt the ink in the conduit and a pump is actuated to transfer ink in a liquid condition. Heaters may be arranged in the ink supply system to maintain thermal gradients and produce convective circulation of molten ink to prevent ink pigments from settling.
PCT International Publication No. WO 89/02575 teaches a hot melt ink supply unit that utilizes a different keyed configuration for each ink color. Ink reservoirs may be formed for each ink color having correspondingly keyed configurations to prevent the possibility of supplying ink of the wrong color to a reservoir reserved for a specific ink color. Solid ink blocks and corresponding reservoirs are configured so that ink blocks having a specific configuration can be received only in reservoirs having that configuration.
European Patent Publication No. 0178886 teaches a solid state ink delivery system wherein solid state ink is stored at a fixed location. A movable imaging head has at least one ink jet and an associated reservoir and may be aligned with the solid ink storage reservoir for transferring ink. Ink transfer is accomplished by melting the ink to a liquid state and permitting it to flow into the imaging head reservoir.
U.S. Pat. No. 4,490,731 teaches an ink dispensing system for a thermal ink jet printer wherein a resistance heating wire traverses the solid ink reservoir and a supply tube connecting the solid ink reservoir with the ink head reservoir. Melted ink is transported through the supply tube by capillary action.
Prior art phase change ink delivery systems, in general, have failed to provide the desired substantially continuous ink flow with a minimum of operator handling requirements and mechanical failures. Ink delivery systems for supplying solid sticks of phase change ink, such as that taught in U.S. Pat. No. 4,870,430, described above, are typically complex and require frequent and alert operator intervention. The ink delivery system of the present invention was therefore designed to provide a substantially continuous supply of different types (e.g., colors) of phase change ink to corresponding print head reservoirs while requiring minimal operator handling and reducing the risks of mechanical failure and operator error.